2 Answers
2

There are two basic usages for the term "crank throw". Both have to do with the crankshaft:

The crank throw is a measure of the distance from the center-line of the main journal to the center-line of the rod journal. This distance could be considered the radius of the crank arm. Twice this measurement should give you the stroke of an engine.

Due to the basic nature of the distance described above, the crank throw is also considered the rod end of the crank shaft. In most V-8 engines, two connecting rods share a "crank throw" and there are four throws to a crankshaft. In most inline 4-cylinder engines, a single connecting rod is on it's own throw and there are also four throws on a crankshaft.

In your examples of either cross-plane or flat-plane crankshafts, both of them have four throws each.

There is such a thing as a split-throw crankshaft. GM used such a crankshaft in their 90° V6 engine. Here is an image of a split-throw crankshaft:

In the image you can see how the journal is split. While being on the same crank arm, the journal (also called a "pin") is split. They will have the same throw (or radius), but are offset from one another slightly.

So how does an engineer decide the number of throws, what are the advantages of each configuration? Is it always the case that V8s have 4 throws and V12s have 3 throws?
– I have no idea what I'm doingFeb 9 '16 at 7:30

@IhavenoideawhatI'mdoing The answer to that is too big for me at the moment. That get's into firing order/degrees on cross plane vs flat plane and on and on. I will say, a V8 Cross plane is almost as well balanced as a V16 flat plane. V8 Cross Plane - 4 throws, A V8 Flat Plane crank would have 2 throws. A V12 could have 6 throws theoretically.
– DucatiKillerFeb 9 '16 at 8:28